The present invention relates to hologram construction and more particularly to a method and apparatus for constructing holograms and holograms made by use of such method and apparatus.
In a known conventional method of transmission hologram construction a single beam of radiation emanating from a single beam source (usually a laser) is split into two separate expanded beams (object and reference beams) of radiation.
The two separate beams, which are conveniently of roughly equal intensity, are combined at the hologram plate which causes high contrast interference fringes to be generated in the region of overlap of the beams at the hologram plate. These fringes may be recorded on a suitable photosensitive material or film (e.g. dichromated gelatin photoresist, silver halide), overlaying the plate, by development and processing of the film. This constitutes the hologram.
This method of construction of holograms suffers from a number of practical problems including a) instability of the system and b) lack of hologram uniformity. Concerning a), it is known that any transitional shift in the relative paths lengths of the two beams during exposure of the photosensitive film has the effect of shifting the interference fringes with the consequence of loss of definition of the recorded fringe pattern, or at the worst, complete loss of the hologram.
Such interference fringe shifts may be caused by vibration within the optical system, the effects of movement of the air in the system, the effects of heating of the system and so on. The effect on the recorded fringe pattern in these circumstances is generally proportional to the duration of exposure of the hologram.
It is possible to minimise these effects by avoiding long beam path lengths, enclosing the system in order to prevent movement of air, using powerful laser beams to reduce hologram exposure times and so on. However, such steps may prove expensive and may not always be entirely successful, especially for the exposure of large area holograms and for photosensitive materials having low sensitivity e.g. dichromated gelatin photoresist, or photopolymers.
Regarding b), it will be apparent that the laser beam intensity profile may take a number of different forms, the precise form depending on the operating mode of the laser and it is known that it is not easy to achieve a uniform exposure of the hologram over the hologram aperture with any of these modes. A solution which is normally adopted, however, is to use the central portion of the Gaussian beam intensity profile of a TEMoo mode of the laser.
It has been found, however, that employing this solution, complete uniformity of exposure is not achieved, and indeed because of considerable energy loss in the outer portion of the beam, longer exposure times are required.
An aim of this invention is to provide a method and apparatus For constructing holograms which substantially reduce the effects of the aforementioned problems.